(1) Field of the Invention
This invention relates to masks and methods for optical proximity correction of pattern images, and more particularly to the correction of line end shortening caused by optical proximity effects.
(2) Description of the Related Art
An important concern in the manufacture of integrated circuit devices is pattern distortion caused by optical proximity effects, particularly as critical dimensions in these patterns decreases. Much work has been done in developing methods to compensate for this pattern distortion.
U.S. Pat. No. 5,807,649 to Liebmann et al. describes a lithographic patterning method and mask set using a phase shift trim mask having mask dimensions increased in block size so as to remove previous exposure defects.
U.S. Pat. No. 5,308,741 to Kemp describes a lithographic method using double exposures, physical mask shifting, and phase shifting masks.
U.S. Pat. No. 5,686,223 to Cleeves describes a lithographic patterning method using multiple exposures to provide for relatively reduced pitch for features of a single patterned layer.
As optical lithography is used to delineate 0.1 micron and smaller features, it inevitably works in a low k1 region. For example, for a 193 nanometer wavelength and a numerical aperture of 0.63 the value of k1, which is equal to the feature size multiplied by the numerical aperture and divided by the wavelength is equal to 0.33. At this low value of k1 the length of lines shortens when the width of the lines is kept at the desired dimension. Conventional solutions to this line shortening have included widening of the line at the line ends. This widening of the line ends takes additional space on the integrated circuit element however, which also limits the density of wiring in the circuit element.
It is a principle objective of this invention to provide a method of forming pattern line elements without line shortening without widening line ends, using double exposure and two masks.
It is another principle objective of this invention to provide a method of forming pattern line openings without line shortening without widening line ends, using double exposure and two masks.
It is another principle objective of this invention to provide a mask set of two masks for forming pattern line elements without line shortening without widening line ends, using double exposure and the two mask set.
It is another principle objective of this invention to provide a mask set of two masks for forming pattern line openings without line shortening without widening line ends, using double exposure and the two mask set.
These objectives are accomplished by forming a modified pattern mask and a cutting mask. A pattern having pattern elements comprising a number of line segments, wherein each of the line segments has one or two free ends which are not connected to other mask pattern elements is to be transferred to a layer of resist. Extended line segments are formed by adding line extensions to each of the free ends of the line segments, thereby forming a modified pattern. A cutting pattern is formed comprising rectangles enclosing each of the line extensions wherein one of the sides of said rectangles is coincident with the corresponding free end of said line segment.
A first mask having opaque regions corresponding to said modified pattern is then formed. A second mask having transparent regions corresponding to the cutting pattern is also formed. The layer of resist is then formed using a first exposure and the first mask. Next the layer of resist is exposed using a second exposure and the second mask for removing any excess line length. When the layer of resist is developed lines having the proper length result.
In another embodiment a pattern having pattern openings comprising a number of line segments, wherein each of the line segments has one or two free ends which are not connected to other mask pattern elements. A cutting pattern comprising rectangles having the same width as said line segments, add length to each of the free ends of the line segments, and overlap the free ends of said line segments is then formed.
A first mask having transparent regions corresponding to the pattern is formed. A second mask having transparent regions corresponding to the cutting pattern is also formed. The layer of resist is then exposed using a first exposure and the first mask. The layer of resist is then exposed using a second exposure and the second mask. When the layer of resist is developed lines openings having the proper length result.
The masks and method of this invention will work for forming images in either positive or negative resist.